A hot runner injection molding apparatus typically includes nozzles that are heated to maintain melt therein at a controlled temperature. The nozzles are typically in contact with a mold component that defines one or more mold cavities. The mold cavities in the mold component are filled with melt that first passes through the nozzles. The mold component is then typically cooled in order to solidify the melt in the mold cavities, thus forming a plurality of molded parts, which are then ejected from the mold cavities.
Because the nozzles are typically heated, and the mold component is cooled at least for a portion of an injection molding cycle, it is desirable to provide a relatively low heat transfer from the nozzles into the mold component. Many nozzle constructions have been proposed in the past to address this issue. An example of such a nozzle construction is shown in U.S. Pat. No. 5,299,928 (Gellert). Gellert proposes a nozzle having a tip through which melt flows and a separate tip retainer piece that retains the tip in place. The tip retainer is made from a material that has a different thermal conductivity than that of the tip. While the configuration proposed by Gellert was an improvement over the prior art, undesirable heat losses can still occur from the tip through to the mold plate.
Another proposed nozzle construction is described in U.S. patent application publication 2003/0118688 (Bazzo et al.) Bazzo et al. describes a nozzle having a tip and an external hollow element around the tip that contacts the mold to form a seal therewith. The tip and the external hollow element do not contact one another directly, and are separated by a gap. There are several problems with this design however. During an injection There are several problems with this design however. During an injection molding operation, melt will accumulate in the gap. During a colour changeover, the accumulated melt in the gap will be difficult to clean out without removing at least the external hollow element from the nozzle, which will extend the time required to effect the colour changeover. Further, the melt in the gap can burn or otherwise degrade after a period of time. The melt can then migrate out of the gap and make its way into the mold cavity, which will negatively impact the quality of the molded article.
U.S. Pat. Nos. 5,545,028 and 5,554,395, both to Hume et al, describe multi-piece nozzle tip assemblies including a tip, a tip retainer and a seal piece. The tip retainer is spaced from the tip along a portion of their respective lengths, however, a region of contact exists where the tip retainer retains the tip. A seal piece is provided between the tip and either the mold plate or the tip retainer, to prevent melt from filling the space between the tip and the tip retainer. However, the tip can still incur heat losses through contact with the tip retainer.
Thus, there is a continuing need for new nozzle constructions that inhibit heat losses out of the tip.